There have been numerous prior art control circuit arrangements wherein the remotely located control unit is operated by signals appearing on the telephone line. These prior art remote units include U.S. Pat. No. 3,865,984 to L. L. Ewing issued Feb. 11, 1975. The Ewing patent teaches a remotely operated door lock system wherein a dc signal is supplied to the control unit via the telephone line. This signal appearing on the telephone line enables the control unit which then activates/deactivates a door lock and applies one of two distinctive audible tone signals to the telephone line, thereby indicating the status of the door lock.
Another prior art remote unit is disclosed in U.S. Pat. No. 3,548,102 to G. Schaum et al issued Dec. 15, 1970. The Schaum patent teaches an arrangement for controlling remotely located heating equipment wherein the control unit is connected to the telephone line and is responsive to the receipt of a predetermined number of ringing signals to activate the heating equipment. Additionally, the control unit returns a distinctive audible tone back to the calling party to indicate the status of the heating equipment.
U.S. Pat. No. 3,702,904 to J. F. Bard issued Nov. 14, 1972 teaches a control unit employing a signal counter circuit. The Bard control unit counts the number of ringing signals received and uses the stored count to control a set of switches, which operate the remotely located equipment. The Bard patent, however, concentrates on the use of a guard interval after the receipt of a valid signal to protect against erroneous activation of the control unit by a subsequent call.
An overall system arrangement is also found in U.S. Pat. No. 4,021,615 issued May 3, 1977 to E. C. James et al, which teaches the provision of apparatus for conserving energy in a hotel/motel building. This is accomplished by equipping each office or room in the hotel/motel with a control circuit to regulate the operation of the energy consuming devices located in each room, these energy consuming devices being primarily the heating and air conditioning equipment. The control circuit is connected to the telephone line associated with the guest room and is responsive to the periodic application of an audio frequency signal to the telephone line to discontinue the operation of the energy consuming device located in that room. The control circuit relinquishes supervision and control of the energy consuming device when the periodic audio frequency signal is removed from the telephone line for greater than a predetermined period of time. The control circuit is also disconnected from the telephone line if the telephone is off-hook and a call is in progress, thus preventing the voice frequency signals of the telephone conversation from erroneously triggering the energy control equipment.
Although the foregoing control circuit arrangements are capable of regulating the operation of remotely located apparatus, they all rely on the connection of considerable additional apparatus to the telephone line to achieve this remote control capability. This additional expense renders many of the potential applications of remote control uneconomical.